Yet even though defensive techniques are now available and effective they can be expensive.

They can also alienate some of a site's target audience - especially teen users who expect more freedom of expression.'There are companies out there that are doing a very good job, working within the confines of what they have available,' said Brooke Donahue, a supervisory special agent with an FBI team devoted to Internet predators and child pornography.

'There are companies out there that are more concerned about profitability.'Also in June, a teen-oriented virtual world called Habbo Hotel, which boasts hundreds of millions of registered users, temporarily blocked all chatting after UK television reported that two sex predators had found victims on the site and that a journalist posing as an 11-year-old girl was bombarded with explicit remarks and requests that she disrobe on webcam.

Officers took control of the teenager's computer and arrested the man the next day, said Special Agent Supervisor Jeffrey Duncan of the Florida Department of Law Enforcement.

The alleged predator has pleaded not guilty to multiple charges of soliciting a minor.

Duncan, one of a half-dozen law enforcement officials interviewed who praised Facebook for triggering inquiries, said: 'The manner and speed with which they contacted us gave us the ability to respond as soon as possible.'Facebook is among the many companies that are embracing a combination of new technologies and human monitoring to thwart sex predators.

Such efforts generally start with automated screening for inappropriate language and exchanges of personal information, and extend to using the records of convicted pedophiles' online chats to teach the software what to seek out.

Another pillar in Facebook's strategy is to limit how those under 18 can interact on the site and to make it harder for adults to find them.

Minors don't show up in public searches, only friends of friends can send them Facebook messages, and only friends can chat with them.

Technology is available for verifying the ages of Web and app users.

One of the providers is Aristotle International Inc, which offers a variety of methods, including having a parent vouch for a child and make a token payment with a credit card to establish the parent's identity.